1. Field of the Invention
This invention relates to castable polyurethane and/or polyurethane/urea elastomer compositions with low hysteresis and improved processing characteristics, including longer pour life, reduced tendency to crack, and reduced presence of toxic free toluene diisocyanate monomer or other mononuclear aromatic diisocyanate monomers. Isocyanate-endcapped prepolymers are employed. Effective processes for the production of such prepolymers and elastomers are disclosed.
2. Description of Related Art
Aromatic polyisocyanates are well known and are widely used in the preparation of polyurethane and polyurethane/urea elastomers. These aromatic diisocyanates generally include compositions such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 4,4'-methylene-bis-(phenylisocyanate) and the like. In the preparation of polyurethane and polyurethane/urea elastomers, the aromatic diisocyanates are reacted with a long chain (high molecular weight) polyol to produce a prepolymer containing free isocyanate groups. This prepolymer may then be chain extended with a short chain (low molecular weight) polyol or aromatic diamine to form a polyurethane or polyurethane/urea elastomer (known generically as polyurethane or urethane). A liquid mixture of prepolymer and curative polymerizes, increasing steadily in viscosity until finally a solid elastomer is formed.
Among the chain extenders or cross-linking agents used, primary and secondary polyalcohols, aromatic diamines, and in particular 4,4'-methylene-bis-(2-chloroaniline) (MBOCA) are most common. The use of MBOCA allows the manufacture of urethane elastomers with good mechanical properties and acceptable processing times.
Although MBOCA is the most widely used chain-extender in the production of castable polyurethanes, it suffers from the disadvantage of decomposition at high temperatures, as well as being quite toxic and Ames positive. The negative features of MBOCA have prompted those in the polyurethane art to investigate alternate materials as chain-extenders. Examples of other amines that have been used include 3,3',5,5'-tetraisopropyl-4,4'-diaminodiphenylmethane and 3,5-dimethyl-3',5'-diisopropyl-4,4'-diaminophenylmethane. While these amines do function as cross-linking agents, the resultant pot life of the polymer mixture is so short that no reasonable processing time for cast elastomers is possible.
Another curing agent used in the manufacture of polyurethanes is methylene dianiline(MDA), which is well-known to those in the art as a good curative if there is only aliphatic diisocyanate present. It has a much shorter pot life than MBOCA. This short pot life is exacerbated by the presence of TDI. There are also toxicity issues related to the use of MDA.
A different chain-extending agent for polyurethanes is 4,4'-methylene-bis-(3-chloro-2,6-diethylaniline) (MCDEA, available as Lonzacure, trademark of the Lonza Corporation). This curative material is reportedly lower in toxicity but faster than MBOCA to react with isocyanates. (See Th. Voelker et al, Journal of Elastomers and Plastics, 20, 1988 and ibid, 30th Annual Polyurethane Technical/Marketing Conference, October, 1986.) Although this curative does react with isocyanate-terminated prepolymers (including TDI-based prepolymers) to give elastomers with desirable properties, such as low hysteresis, they have a tendency to crack when undergoing polymerization.
The amount and presence of free, unreacted TDI monomer has other deleterious effects on the processing and manufacture of urethanes. A major problem with mononuclear aromatic diisocyanates, such as toluene diisocyanate, is that they are toxic and because of their low molecular weight tend to be quite volatile.
U.S. Pat. No. 5,077,371 to Singh et al discloses a prepolymer that is low in free TDI. U.S. Pat. No. 4,182,825 to Jackle also discusses polyether based prepolymers made from hydroxy terminated polyethers capped with toluene diisocyanate with the amount of unreacted TDI substantially reduced. These prepolymers can be further reacted with conventional organic diamines or polyol curatives to form polyurethanes. When combining the teachings of this patent with the use of MCDEA as a chain extender, the resulting solid elastomer goes through a gel stage having low strength which can allow cracking of the polymerization mass to occur. Conventional TDI prepolymers with higher levels of free TDI also yield the same unsatisfactory gel state.
Surprisingly, it has been found that certain prepolymers prepared with both TDI and an aliphatic diisocyanate can be used with MCDEA to give elastomers with both reduced propensity to crack and very low hysteresis. The prepolymers also provide extended pour life and reduced presence of toxic free TDI. An example of aliphatic diisocyanate would be a mixture of the three geometric isomers of 1,1'-methylene-bis-(4-isocyanatocyclohexane), abbreviated collectively as "H12MDI." One such mixture of isomers is available commercially as Desmodur W (trademark of the Miles Corporation).
These results are surprising. The low hysteresis of these compositions is particularly surprising in light of the poor (high) hysteresis ordinarily observed when H12MDI is employed in polyurethane compositions.
It has also been observed that using a blend of MCDEA with a small amount of MBOCA sharply reduces the weakness of the gel stage, resulting in a desirable reduction of propensity to crack. When MBOCA is incorporated into the curative, aliphatic diisocyanate need not be incorporated into the prepolymer to achieve a reduction in propensity to crack.
These results will become clear in the following description and illustrative examples.